1. Field of the Invention
This invention generally relates to semiconductor optoelectronic circuit design and manufacturing. More particularly, it relates to an apparatus and method for precisely aligning a photolithography mask with the orientation of the crystal lattice of a semiconductor wafer.
2. Description of the Related Art
The semiconductor industry has made great strides toward achieving smaller, more densely packed and faster optoelectronic circuits with each successive generation. As a result, it has been increasingly important to refine the tolerances of each step of making an optoelectronic circuit in order to achieve the highest precision circuit possible.
Semiconductor substrates are often used for precisely aligning optical fibers. V-grooves etched into the substrate operate as a guide for the optical fibers that are placed therein. Semiconductor processing technology for transforming a semiconductor substrate into one or more optoelectronic devices involves a complex sequence of processing steps which must be accurately performed to produce properly functioning circuits. One of the most critical steps in the process is generating accurate alignment marks on the semiconductor substrate prior to etching V-grooves into the substrate. When etching a substrate, an etchmask or series of etchmasks, each with a predetermined pattern, is used to permit selective portions of the semiconductor substrate to be etched away while leaving other areas of the substrate intact.
When an etchmask is used to etch a structure such as a V-groove into a semiconductor substrate, a certain amount of undercutting will occur. Using an example of silicon (Si) as the substrate 50 and silicon nitrate (SiNx) as the etchmask 52, it is desired to align the etchmask 52 with the crystal lattice orientation of the silicon substrate 50 as precisely as possible. Referring to FIG. 1, the etchmask 52 has at least one window opening 54 having a certain predetermined width W. This width W is used to achieve a particular desired depth D in the V-groove 56. Precise alignment of the etchmask 52 with the crystal lattice orientation of the substrate 50 results in repeatable and definable etch depth D of the V-groove 56. If the etchmask 52 is properly aligned with the crystal lattice orientation of the substrate 50, symmetrical undercutting 58, (i.e. the same amount of undercutting under all sides of the window opening 54), of the etchmask 52 will occur as shown in FIG. 1. Misalignment of the etchmask 52 and the crystal lattice orientation of the substrate 50 will result in uneven undercutting of the etchmask 52.
The crystal lattice orientation of the substrate has a substantial effect upon the etching process since the etching tends to follow the lattice orientation. As a result, if the etchmask is not properly aligned with the crystal lattice orientation of the substrate, the pattern etched upon the substrate will not be the same as the desired pattern. Accordingly, properly aligning the etchmask with the crystal lattice orientation is critical to achieving more exact tolerances in semiconductor optoelectronic circuit fabrication.
The semiconductor industry has long realized the need for aligning the crystal lattice orientation with the etchmask. Semiconductor substrates, generally shaped as a circle, are provided with a "flat" wherein a portion of the circle is cut as shown in FIG. 2. The flat acts as a guide for aligning the etchmasks thereupon. The equipment used to determine the crystal lattice orientation and mill the flat is accurate to plus or minus 0.5 degrees with respect to the crystal lattice orientation. This 0.5 degree tolerance is unacceptable for certain applications, including the precise alignment of optical fibers in optoelectronics.
Accordingly, there exists a need for a simple procedure for accurately aligning an etchmask with the crystal lattice orientation of the semiconductor substrate.